1. Field of the Invention
The present invention relates to a process for producing 2-methyl-l-naphthol, which is useful as an intermediate for producing medical pharmaceuticals, such as menadione (Vitamin K.sub.3) and Vitamins K.sub.1 and K.sub.2.
2. Description of the Related Art
Known processes for synthesizing 2-methyl-1-naphthol are those utilizing as starting materials 1 1-naphthol, 2 2-methylnaphthalene and 3 1-tetralone.
The process with the above 1 comprises permitting methanol to add to 1-naphthol in gaseous phase in the presence of (i) Al.sub.2 O.sub.3 (U.S. Pat. No. 3,993,701, Switz. Pat. No. 598170), (ii) Fe.sub.2 O.sub.3 or Cr.sub.2 O.sub.3 (Chem. Pharm. Bull., 24(9), 2199 (1 976) (iii) a combination of cerium oxide and an oxide of a metal such as antimony, germanium, tin or magnesium (Japanese Patent Application Laid-open No. 47958/1975) or like catalysts. This process has the drawbacks of requiring, generally, a high temperature of 325.degree. to 425.degree. C. and generation of a large amount of by-products. There has also been reported an attempt to methylate the naphthol with a methylation agent such as an organic lithium and methyl iodide or dimethyl sulfate (Indian J. Chem., Sect. B, 21B (5), 474 (1982); J. Org. Chem., 53 (22), 5345 (1988)). This is an uneconimical process because of low yield.
The process with 2 2-methylnaphthalene comprises converting it with formic acid, acetic acid or propionic acid to the corresponding naphthyl ester by electrochemical reaction and then hydrolizing it into 2-methyl-1-naphthol (DE 2434845). This process also suffers a low yield and cannot be said to be suitable for commercial production.
An example of the process with 3 1-tetralone comprises converting 1-tetralone to 2-methyl-1-tetralone and then dehydrogenating it by heating in the presence of Pd/C (Japanese Patent Application Laid-open No. 65235/1983), while another one comprises the successive steps of converting 1-tetralone to 2-benzoyloxymethylene-1-tetralone, subjecting it to hydrogenolysis in the presence of Pd/C and cyclohexene and isomerizing the resulting product (Chem. Ind., 14 , 5052 (1981)). However, there is a limit to the yield of monomethylation of 1-tetralone with the former process, and the latter is uneconomical because of long reaction paths.